This invention relates to a filter cartridge construction suitable for filtering a slurry composition comprising solid particles in a liquid carrier. More particularly, the present invention relates to a filter cartridge construction for filtering a particle-containing paint composition or slurry for making a particle-containing paint composition including a pleated or spirally wound filter element formed from a woven fabric having uniform sized openings and spacer layers on each filter layer surface. The present invention also provides a process for filtering a particle-containing paint composition or a precursor slurry useful for forming a particle-containing, paint composition.
At the present time, compositions comprising a liquid carrier and solid particles such as a particle-containing paint, herein after "particle-containing paint composition" are filtered to remove over-sized particle by utilizing a depth filter or a bag filter. The depth filter is generally formed from a layer or multiple layers of various fibrous filter media which are wound or blown around a central hollow core that accepts the filtrate. When filtering particle-containing paint compositions, the purpose of the filter is to (a) retain only the large undesirable metal particles which can form imperfections in the final dried paint surface which resembles an orange peel surface and (b) allow the smaller sized desired particles to pass through the filter. Filtration of the particle-containing paint composition is effected up to the point the filter becomes plugged with over-sized particles and smaller particles. The gradual retention of the desired size smaller particles by the filter causes a significant reduction in useful filter life. When filtering particle-container paint compositions, a typical useful life of a presently available depth filter for removing over-sized particles is between about 30 and about 60 minutes. When the filter is plugged, a pressure drop larger than desired across the filter is obtained which renders it useless for its intended purpose. The plugged filter must be disposed of in an environmentally acceptable manner. The disposal cost is a direct function of the useful life of the filter.
Woven screen filters formed from woven stainless steel fibers presently are employed in steam lines or in chocolate manufacturing to remove undesirably large particles from either the steam or the chocolate. These screens generally are unsupported screens. Screen filters formed from woven polymeric fibers such as polypropylene fibers are also available. These screens can be made with openings of varying size depending upon the closeness of the weave utilized to form the fabric. Varying weave patterns can be utilized to form the fabric filter. Square weaves comprise a simple over-and-under pattern, providing a straight through flow path with a relatively high percentage of open surface area. The square weave fabrics are used in sifting screens, high flow rate filters, carrier belts, glare-reduction screens, plankton nets, micro-strainer screens, disc sector covers and strainer bags. These screens also can be pleated to be used to filter fuel or hydraulic fluids. This screen also is utilized alone and in a flat configuration to test the efficiency of prior art processes for filtering particle-containing paint compositions. The particle-containing paint composition is deposited on one surface of the screen and is passed through the screen to determine whether there are particles retained on the screen which should have been retained in the prior-used paint filtration process. Other weaves for fabric filters include twill weave, plain reverse dutch weave and taffeta weave fabrics.
A surface or screen filter will retain virtually 100% of the particles or contaminants for which it is rated. The media used in the surface or screen filter typically has a high pressure drop and low "dirt holding capacity" or throughput because of its high retention efficiency. The normally used medium in the surface or screen filter comprises a polymeric microporous membrane or a woven screen. Particles are retained by size exclusion primarily on the surface of the screen filter which has a controlled pore size rather than within the depth of the filter. Particles smaller than the controlled pore size tend to be trapped within the media of the surface filter. However, as a result of the controlled pore structure, they provide more predictable filtration than depth filters. For the surface of a screen filter to be economical, the media is pleated to obtain a large filtration surface area. Wound depth filters and surface or screen filters are utilized in series in separate housings to effect the desired level purification.
Particle-containing paint compositions generally comprise a polymer such as a polyacrylic such as polyacrylic acid, a latex or the like; a solvent for the polymer; a colorant composition such as a pigment and light reflecting particles such as metal or mica or mixtures thereof. The particle-containing paint composition contains solid light-reflecting particles such that the viscosity of the final paint composition typically is between about 150 and 800 centipoises (cp). The light reflecting particles have a largest dimension sufficiently small so that when the paint is dried as a coating on a substrate, it presents a smooth surface. The largest dimension particle which is desired to be removed from a paint composition while permitting the retention of smaller particles in the composition depends upon the use to which the paint is applied. Typically the largest particle desired will range from about 10 to about 100 microns. Typically-used light reflective particles in particle-containing paint compositions include a metal such as aluminum, silver or gold or mica or mixtures thereof. Generally, a particle-containing paint composition is formed in a multi-step process including a step of mixing light reflective particles, usually as flakes with a liquid carrier such as water, water containing low concentrations of an aromatic solvent or an aromatic hydrocarbon such as xylene or toluene or, methyl ethyl ketone or the like which is also a solvent for the polymer constituent of the paint composition. This composition is referred to herein as the particle loading slurry composition. The loading slurry composition is highly viscous to the extent that it is difficult to pour from an open container even when the container is inverted with the opening down. After the polymer, solvent and colorant constituents have been mixed, the appropriate volume of the particle loading slurry composition is mixed therewith to form the particle-containing paint composition.
In the past the procedure which was followed, included mixing the paint composition for several days while color and viscosity were adjusted. The final product was filtered as it was pumped into shipping containers. Quality requirements were not stringent and most filtration was done using resin bond cartridges and later felt bag filters. During the last few years formulations and painting techniques have changed. Paint is now a much more high solids slurry with fewer coats of paint being needed to produce final surface color. In addition, the thickness of paint layer has decreased to approximately 25 microns which has precipitated more stringent requirements for acceptable contaminant size and quantity. Existing techniques for filtration no longer are effective especially when all filtration is confined to the filling step wherein the flake is added to the final paint composition. Presently available depth filter cartridges characterized by very flat retention curves are now plugging as they remove very small percentages of the now highly concentrated pigments and light reflecting particles. This plugging problem is more predominant with the particle-containing paint because of the non specific retention obtained with the less retentive grades of depth cartridges. This result is common to all graded density depth cartridges.
Theoretically, the most desirable method to overcome this problem is to prefilter the slurries before they are added to the final mix followed by a final filtration during the filling. However there has been no filter available that can process these high solids metal solutions economically. Excessive amounts of satisfactory particles are undesirably removed while rapid filter plugging is experienced.
It would be desirable to provide a filter cartridge construction having an extended useful life as compared to present filter cartridge constructions utilized to filter slurries such as particle-containing paint compositions or the particle loading slurry composition for particle-containing paint compositions. Furthermore, it would be desirable to provide such a filter cartridge which selectively permits passage of particles of a desired size through the cartridge while selectively retaining particles larger than that of the desired size so that retention of desired sized particles is minimized or eliminated. Such filter cartridges, would provide substantial improved economic benefit from the standpoint of increased satisfactory throughput fluid volume capacity and low disposal costs.